How bathroom sink plumbing works, including a diagram of the drain plumbing assembly
A bath sink typically has two fixture holes on either 4-, 6-, or 8-inch centers. The wider types are meant to receive a split-set faucet, with faucet handles separate from the spout. The 4- or 6-inch holes may receive either a center set or a single-lever faucet. Flexible supply tubes carry water from shutoff valves at the wall to threaded tailpieces on the base of the faucet.
The drainpipe that runs through a bathroom sink usually is fitted with a pop-up stopper that raises and lowers when you pull up or push down on a knob or handle just behind the faucet body. The knob is actually the head of a lift rod fastened to a clevis, which is a connecting bar. The clevis connects to a pivot rod-and-ball assembly that consists of a rod that runs through a rubber pivot ball and slopes slightly uphill to the tailpiece of the stopper.
Pushing the knob and the lift rod down causes the pivot rod to push the stopper up; pulling the knob up causes the pivot rod to pull the stopper down. If you want to remove the assembly, you may be able to pull it right out. Or, you may have to twist it to unhook it from the clevis. (For more information, see How a Pop-Up Drain Stopper Works.)
The sink drain has a flange that is sealed to the sink hole with plumber’s putty. This flange is screwed into the drain body, which is tightened to the underside of the sink bowl with a locknut.
The tailpiece, which may be fitted to a pop-up stopper, attaches to a drain trap by means of slip-joint couplings. A sink trap remains filled with water so that sewer gases can’t enter the room; the trap is connected to a threaded nipple inserted in a T in the drain line. An escutcheon trim hides the connection.
A mechanical pop-up stopper is operated by a system of levers and rods. If this isn’t working properly, the solution is usually just a matter of adjusting the clevis screw or the position of the pivot rod.